I'm using the 60mb/sec CF cards I used with my 5D MK II and my 7D. They are only 16GB, and 64 GB would be better.
I bought a 30mb/sec class 10 SDXC card earlier this week on a Amaxon.com special, so I will not have to change my backup card frequently. Right now, I'm writing jpegs to the SDXC and raw to the CF, but that's just a test. I could write a lot of jpegs to a 64GB card.
i'll try writing raw to both and see how much that slows it down.